draft-rosenberg-mmusic-sdp-offer-answer-00.txt Jonathan Rosenberg dynamicsoft IETF 52

History RFC2543 had appendix B, which specified SDP usage As bis evolved, this section become more independent and moved towards a well-defined offer/answer model Agreement at IETF 51 in both mmusic and sip to extract to separate doc and progress in mmusic NB: sip-bis depends on this draft!! Thus, this draft is needed for 3gpp R5

Open Issues Allow changes in the media type (audio/video) of a stream? –Example usage is voice->fax –Only really reasonable if SIP issue #24 allows for replacing streams If you change audio to message, its same as a new stream in the old slot Proposal is to allow –I say yes Offerer prepared to receive media when it sends offer, even in bidirectional streams –Flemming argues it should be prepared to both send and receive –Seems academic point to me

Multiple streams of same type What is meaning when there are multiple streams of the same type? –Spec says to send a copy of your media on each, and mix media received on each –Clearly specific to audio, doesn’t make sense for Video or IM Cases where there are multiple media sources/sinks

Proposal Model is –Element has sources (green) and sinks (blue) for each type –Streams are uni or bi-directional Requirement –Media received on a stream MUST get sent to one or more sinks –Sources MUST go to one or more stream –When more than one source transmits on a stream, it must be “combined” in some implementation specific way –When more than one stream transmits to a sink, it must be “combined” in some implementation specific way –Mapping of sources/sinks to streams beyond the above rules is local policy f1f1 f2f2 f 1 and f 2 are surjections SDP source sink

Synchronizing Codec Changes A and B are in a session X, Y codecs A offers B new SDP with new codecs Y,Z –B answers with Y,Z Issue: when can A and B cease being prepared to use X? –If there is no overlap – its easy – when media from new codec arrives If there is overlap –Proposal I: when a non-overlapping codec is received, OR 1 minute passes Timer based stuff ugly –Proposal II: answerer includes SN of first packet sent after answer was sent Offerer can stop when it receives that packet Only works for answerer

Unidirectional codecs in a bidirectional stream Motivation –PC phone calls media server –PC phone can send DTMF, can’t receive –MS can receive DTMF, not send –Would like PC to use some codec when sending voice, switch to rfc2833 for DTMF Question –How to represent? Current text –Offerer omits rfc2833 entirely –Answerer adds rfc2833 –Semantic: codecs not in offer, added to answer, on a sendrecv stream, are recv only –Problem: makes interpretation context dependent Breaks 3pcc

Other proposals Offerer includes rfc2833 anyway, even though it can’t receive it Answerer can’t insert it unless it can BOTH send and receive it –My preference Rfc2833 in NEITHER offer or answer –MS adds an extra m line through a new offer –Extra m line is recvonly with rfc2833 –Use FID to group them –Complex Others?